1. Field of the Invention
This invention relates to a protective device for eyes, particularly goggles or face shielded helmets for sports to be used in skiing, snowboarding, ice skating, cycling, motocross motorcycling etc. as well as industrial goggles or face shields to be used in a building site, civil engineering work and the like.
Especially, the invention relates to goggles, face shielded helmets and face shields excellent in a function of protecting eyes from impact, rain, snow, dust, or the like and hardly fogged in the double lens even in the case of sweating or ambient temperature fluctuation.
2. Description of the Related Art
In sports accompanied with speeds such as skiing, snowboarding, ice skating, cycling and motocross motorcycling, goggles and face shielded helmets for covering both eyes have been widely used for protecting the wearer's eyes from unexpected accident such as a fall and a crash and also for keeping a clear visibility at the time of raining, snowing, or the like.
Further, goggles and face shields have been widely used for protecting the wearer's eyes in so-called dusty work sites of, for example, demolition of buildings, earth and sand collection and stone crushing, tunneling, mining, cement production, and steelmaking.
These goggles and shields are called as a single lens since they typically have a single lens.
For goggles, face shielded helmets and shields, it is indispensable to have a damage-preventing function for protecting eyes from a fall or a crash and a foreign substances-shutting function for protecting eyes from rain, snow, and dust. However, these goggles and shields have been insufficient in antifogging function for preventing lens from fogging due to sweating or ambient temperature fluctuation in use.
The foreign substances-shutting function depends on the structure itself of covering both eyes, and therefore this function does not depend on the property of a resin to be used for the lens. On the other hand, the damage-preventing function depends on the property of the base material of the lens, such as bending rigidity and impact-resistant strength. Since polycarbonates are recognized as a material having good stiffness, a strong toughness and difficulty of being bent, polycarbonate lenses have been generally used.
With respect to the antifogging function of goggles and shields, a method for forming an antifogging coating on the inside of the lens, that is the side facing to eyes, has been employed. However, any antifogging agent has a limited capacity in absorbing moisture and once a prescribed amount of moisture is absorbed, droplets are likely to be formed on the antifogging coating, resulting in the lens being fogged.
Moreover, since the goggles and shields forms a kind of sealed space between the skin and the lens facing to the eyes, the moisture such as sweat supplied constantly from the skin tends to filled in that space. Therefore, it is inevitable to cause fogging sooner or later while wearing the goggles or shields, and it is thus difficult to give them a semi-permanent antifogging property.
For such a limited antifogging function, a double lens structure in which two lenses arranged approximately in parallel are fixed by gaskets placed on the circumferential part of the lenses, that is called a double lens, has been proposed.
For the lens facing to the outside, that is a front lens out of two lenses composing the double lens, polycarbonate resins have been commonly used as before in order to obtain the damage-preventing function.
On the other hand, for the lens facing to the eyes, that is a back lens, acyl cellulose resins having a moisture absorbing property have been used. Acyl cellulose, especially, propyl cellulose is preferably used.
In the double lens structure, the space formed by the two lenses and the gasket functions as a temperature buffer zone, so that the cooling inside of the goggle can be moderated even if it is exposed to the cold outside air.
Accordingly, the water vapor enclosed in the space between the lens and the skin is less likely to cause a dew condensation on the back lens, and even if a dew condensation is occurred, since they are absorbed in the propyl cellulose, fogging of the lens facing to the eyes of the double lens is less generated as compared with the goggles with the single lens structure.
However, since the acryl cellulose type resins generally have high moisture permeability, the water vapor enclosed in the space between the lens and the skin through a back lens made of propyl cellulose is easily permeated to the inside space surrounded with the front lens, the back lens and the gasket and this causes a problem that the inside of the front lens, that is the back curve side of the front lens, is fogged when a wearer goes outside like an environment at a temperature of about −5° C. in a skiing area from inside of a warm room.
To deal with this problem, it is proposed to form an antifogging coating on the back curve side of the front lens (Japanese Patent Application National Publication No. 2002-505157)